The present invention relates to an optical receiver for use in optical communication system, and more particularly an optical receiver for a system such as an ATM-PON optical subscriber system in which burst transmission is carried out.
A configuration example of the conventional optical receiver corresponding to burst transmission is shown in FIG. 1. The 2R regenerator 1 shown in this example is the one published in B-1034 at the 1996 General Conference of the Communications Society of the IEICE (Institute of Electronic, Information and Communication Engineers).
Also, the bit synchronizer 2 is the one published in B-844 at the 1996 General Conference of the Communications Society of the IEICE. The above-mentioned 2R regenerator 1 is an optical signal receiver having functions of waveform regenerating and waveform reshaping against received light, in which a received optical signal is converted to a voltage signal through a photodiode 10 and a pre-amplifier 11. A binary signal is output after an ATC circuit 12 discriminates the signal in an amplitude direction.
ATC circuit 12 is provided with an automatic threshold value control function to discriminate and detect level xe2x80x981xe2x80x99 and level xe2x80x980xe2x80x99 by setting a threshold level in the middle of the received signal. A reset circuit 13 is provided for resetting the preset threshold value of ATC circuit 12.
Bit synchronizer 2 inputs the output signal of this 2R regenerator 1, to sample at multiphase timing using a system clock CLK being externally input. Further, using this sampled result, bit synchronizer 2 detects a rise change point (edge) and a fall change point on both sides of the input data to select the center phase therebetween to output as the optimal phase for data sampling.
More specifically, the output of 2R regenerator 1 is commonly input to a plurality of timing adjustment circuits (delay line) 200-20n. The outputs of timing adjustment circuits 200-20n are further input to the corresponding flip-flop circuits 210-21n. Here, the output data of 2R regenerator 1 are latched in multiphase at the inputs of flip-flop circuits 210-21n at the timing determined by the external clock CLK.
The outputs of flip-flop circuits 210-21n are input to the corresponding shift registers 220-22n, as well as to an edge detection circuit 20.
Edge detection circuit 20 detects change points of the input signal, namely both a rise edge and a fall edge of the output of 2R regenerator 1, from the outputs of flip-flop circuits 210-21n. 
Accordingly, based on the detection result of the rise edge and the fall edge, a phase selection circuit 21 controls a selector 22 so as to select a shift register output corresponding to the data being latched at the timing of the center phase of the detected edges.
Here, in the above-mentioned configuration, the signal output of 2R regenerator 1 is inclined to fall into the state of varied pulse width caused by either nonlinear circuit operation or an error in setting a discrimination threshold value by an ATC circuit 12.
There may also be a case that the pulse width itself of the optical signal data received from the transmission side to input to 2R regenerator 1 has already been varied for such a reason as the characteristic of a laser on the transmission side. Therefore, bit synchronizer 2 is required to cope with such pulse width degradation in the input data.
Considering this issue, in the conventional configuration example shown in FIG. 1, a measure has been taken to cope with pulse width degradation by detecting the both edge sides, or a rise edge and a fall edge of the data, to determine the center phase therebetween as a discrimination phase.
However, the method of detecting both edge sides has a limit from the viewpoint of tolerance against the pulse width deterioration.
One example resulting from varied data width is shown below: Consider a case of an NRZ signal data having a pattern of xe2x80x980100xe2x80x99 being input with a duty ratio of 170%, and a case of a data with a pattern of xe2x80x980110xe2x80x99 being input with a duty ratio of 70%. It is not possible to distinguish these two cases as shown in FIG. 2 which illustrates a problem in the conventional optical receiver configuration.
Namely, as shown in FIGS. 2(a) and 2(b), for the cases of receiving xe2x80x980100xe2x80x99 and xe2x80x980110xe2x80x99 with a duty of 100%, consider that these data width are deteriorated resulting in such conditions that the respective data have duties of 170% and 70% as shown in FIGS. 2(c) and 2(d). In such cases the NRZ signal patterns become identical.
As a result of edge detection in edge detection circuit 20, there are two sets of the rise timing in phases xc3x82 and xc3x83, and also two sets of the fall timing in phases xc3x815 and xc3x816.
Therefore, the timing position of phase xc3x89 being located at the center of those two positions is determined as the sampling position for each bit. As a result, in the case shown in FIG. 2(d), it is determined as xe2x80x980xe2x80x99 in the third bit, thus producing an error.
In order to circumvent the above-mentioned problem, there has been taken in such a conventional method a measure of restricting the tolerable pulse width variation value within the range of 50% to 150% for signals being input to bit synchronizer 2.
Accordingly, it is an object of the present invention to provide an optical receiver having an extended tolerable variation value of input signal pulse width for use in a system such as ATM-PON optical subscriber system in which burst transmission is carried out.
As a basic feature, the optical receiver to attain the object of the present invention includes; an optical signal receiver for receiving an optical signal in burst transmission to regenerate and reshape waveform of the received optical signal; and a bit synchronizer for detecting a rise change point and a fall change point of the signal being output from the optical signal receiver using an externally-input clock to determine a discrimination phase according to the detected change points, and to discriminate to output the received optical signal at the timing of the determined discrimination phase.
The above-mentioned optical signal receiver further includes an amplitude detector for detecting the received optical signal intensity, and also the above-mentioned bit synchronizer further includes a phase selector for selecting the discrimination phase timing corresponding to the received optical signal intensity detected by the amplitude detector.
Moreover, as a preferred embodiment of the present invention, the optical receiver includes; an opt-electric converter for converting a received optical signal into an electric signal; a pre-amplifier for amplifying the electric signal; an amplitude detector for detecting amplitude intensity of a signal output from the pre-amplifier; a main amplifier for amplifying the pre-amplifier output signal as well as for setting a discrimination threshold value to discriminate in the amplitude direction; a multiphase sampler for splitting an output signal of the main amplifier into a plurality of branch signals to apply mutually different delays to the each branch signal and thereafter to discriminate an input signal using the identical externally-input clock; a change-point detector for detecting a rise change point and/or a fall change point of a signal output from the multiphase sampler; a selection phase controller for outputting a control signal to determine a signal discrimination phase according to both the change point detection result from the change-point detector and a value detected by the amplitude detector; and a selection signal output portion for outputting a signal discriminated by the discrimination phase corresponding to the control signal being output from the selection phase controller,
The above-mentioned selection phase controller generates a control signal for determining a signal output from the selection signal output portion according to the reception signal intensity detected by the amplitude detector.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.